The present invention relates to an integrated circuit breaker/starter for a motor.
In the field of motor control, it is known to control the operation of a motor (e.g., to start or stop the motor) using a contactor, which is a three pole switch which is electrically operated by a (usually) continuously energized solenoid operating coil. It is also known to provide thermal protection, i.e., overload protection, to a motor against overload conditions using a motor overload relay. Overload conditions occur when equipment is operated in an electrically undamaged circuit in excess of normal full-load rating, or when conductors carry current in excess of rated ampacity. Overload conditions persisting for a sufficient length of time will damage or overheat the equipment. Overload conditions do not include faults which require instantaneous protection such as a short circuit or ground fault or a loss of a phase. The terms "overload," "overload protection" and "overload relay" are defined in the National Electrical Manufacturers Association (NEMA) standard ICS2, which is herein incorporated by reference. Typical overload relays have been implemented using bimetal relays, and more recently using electronics and current transformer sensors. A conventional motor starter is typically implemented by a combination of a contactor and a motor overload relay.
Overload conditions result in a cumulative heating effect in motor circuits, and subsequently a cooling effect after the motor circuit is deenergized, such as with an overload relay. Therefore, the length of time that a motor can operate before overheating under overload conditions will vary if the motor is energized and deenergized too frequently. This cumulative heating and cooling effect is known as thermal memory, i.e., operating memory as defined in NEMA standard ICS2.
Typical overload relays, such as bimetal relays, compensate for thermal memory of the motor mechanically through the thermal memory of the bimetal components within the relays themselves. However, thermal memory, i.e., the cumulative heating and cooling effect, changes between motor applications. Therefore, a bimetal relay must be matched to a particular motor and cannot be used to provide overload protection for more than one motor application.
Electronic devices, e.g., electronic overload relays or electronic trip units, can compensate for thermal memory through software algorithms. The algorithms have adjustable parameters that can be changed from one motor application to another. However, unlike the bimetal relays, the ability to compensate for thermal memory is lost in prior art electronic devices when power is interrupted.
To protect an electrical motor from electrical overload conditions, it is known to use a circuit breaker in combination with a motor starter. Motor control centers and combination starter panels both use motor combination starters. There are typically two types of circuit breakers used in motor starter applications. The first is an "inverse time" general circuit breaker, and the second (more common) type is the "instantaneous trip" only circuit breaker, which provide instantaneous protection from faults such as short circuits, ground faults or a loss of a phase. The instantaneous trip circuit breaker is more typically used in motor applications due to cost considerations, and because the use of an inverse time circuit breaker provides more protection than is typically needed. Further, inverse time circuit breakers are not typically configured for motor protection, as motor protection requires different trip times than typical circuit breaker applications.
A typical motor application circuit is shown in FIG. 1. The circuit is connected between lines L1 and L2 and includes a normally-closed stop switch 10, a normally-open start switch 12, a contactor coil 14, and a conventional overload relay 15. The contactor coil 14 is energized or de-energized appropriately to operate contactors in a three-phase system, where each of three phase lines A, B, and C has a circuit breaker 16a, 16b, and 16c, respectively, contactors 14a, 14b, and 14c, respectively, and motor overload protection 18a, 18b, and 18c, respectively. The circuit breakers 16a, 16b, and 16c are typically implemented by instantaneous trip circuit breakers.
It would be desirable to consolidate the circuit breaker instantaneous trip with a motor starter overload protection. It would also be desirable to be able to vary or reconfigure the circuit breaker trip time for different motor applications. It would further be desirable to prevent the circuit breaker from tripping during a motor overload condition and to be able to provide a substantially continuous power supply to the motor electronics so that the occurrence of an overload condition and thermal memory can be remembered.